The first stage of planet building is collisional
coagulation of dust aggregates in the solar nebula. Although
the importance of the collision is well recognized, there
has been a few attempt to simulate collision of large (~
cm sized) aggregates. Here I report the results of
numerical simulations of dust aggregate collisions using SPH
hydrocode. The essential parameter is the mechanical
strength of dust aggregates, since it is expected that the
strength of aggregates varies extensively from 103\,{\rm
dyn\,cm}-2 (estimated tensile strength of comet SL9:
Greenberg et al. 1997) to 107\,{\rm dyn\,cm}-2
(tensile strength of commercial polycrystalline ice). It has
been found that sticking occurs under limited conditions, in
which tensile strength is larger than compressive strength
of aggregates. This results suggest that the aggregate
coagulation processes are highly material dependent, and
collisional growth proceeds in the limited locations in the
solar nebula.